sedimentary
Chert (and/or Chalcedony)
Microcrystalline Silica (SiO2)
Hardness: 6.5-7 (Mohs); Color: tan, cream, off-white, and grey; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.6 - 2.65.
- Hardness
- 6
- Color
- tan, cream, off-white, and grey
- Luster
- waxy to dull
Identified More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 6.5-7 (Mohs); Color: tan, cream, off-white, and grey; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.6 - 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (such as diatoms or radiolaria) or by the chemical precipitation of silica in groundwater within limestone or chalk beds. Prevalent in sedimentary layers ranging from the Precambrian to Cenozoic eras.
Uses & applications
Historically used for stone tools and arrowheads due to its sharp edges. Modernly used in construction aggregate, as a gemstone (when colorful, like jasper/agate), and for making fire with steel.
Geological facts
Chert is so durable that it can survive long periods of weathering. Because it breaks with a conchoidal fracture like glass, it was the primary material for 'flint knapping' by prehistoric humans to create survival tools.
Field identification & locations
Identify in the field by its waxy texture and shell-like (conchoidal) breaking pattern. It will easily scratch glass but cannot be scratched by a steel knife. Commonly found as nodules in riverbeds or within limestone outcrops.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone (Quartz-rich)
Arenite (SiO2)
sedimentary
Sandstone (with man-made markings)
Arenite (composed primarily of Quartz, SiO2)
sedimentary